Arc Chart in Tableau

Like most people with eyes and a heart I was moved by the gun death visualization by Periscopic. It's powerful and amazingly well done. So using the data from my Supreme Court visualization I attempted to create a similar arc chart in Tableau. This visualization displays tenure by Supreme Court Justice broken out by political party. The height represents the age of the Supreme Court Justice at the time of their appointment (taller = younger) and the color represents the year they were appointed (lighter color = earlier). You can filter to show the change in justice tenure over time.

See details after the break on how to create an arc chart in Tableau.

To create an arc chart you first need to create to create concentric circles for every Justice. So I plotted 60 points along the perimeter of a circle. This created an X and Y coordinate for every point on a circle. The formula I used in Excel to create these points was the following:
  • X1 = 1 + SQRT(2) ×  SIN(A1 × PI() / 180)
  • Y1 = 1 + SQRT(2) × COS(A1 × PI() / 180)
This returns a circle for every Supreme Court Justice.

In order to create arcs you need to alter the width (X1 or tenure) and the height (Y1 or age at appointment) of the circle in order to alter the curvature. So then multiply each point on the circle by your variable: X1 × tenure (days) and Y1 × age (years) at appointment. I used the inverse of age at appointment so that younger is taller and older is shorter. This then distorts the circle and produces arcs.

Then I split the chart into two: one appointed by Democrats and another appointed by Republicans. And then I cut each of the charts in half by truncating the Y-axis. Finally, I colored by party and formatted the background, font, and axes.

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